The present invention relates to a process for producing aluminum having high purity, and more particularly to a process for treating aluminum containing eutectic impurities, such as Fe, Si Cu and Mg, and also peritectic impurities, such as Ti, V and Zr, by resorting to segregation solidification to obtain aluminum having lower contents of eutectic impurities and peritectic impurities and a higher purity than the original aluminum.
Throughout the specification and the appended claims, the term "eutectic impurities" refers to the impurities which undergo a eutectic reaction with aluminum, and the term "peritectic impurities" refers to the impurities which undergo a peritectic reaction with aluminum.
When aluminum containing both eutectic impurities and peritectic impurities is melted and then subjected to equilibrium solidifcation, the impurity concentration Cs of the resulting solid aluminum fraction is theoretically represented by the following equation. EQU Cs=koCo(1-fs).sup.ko-1 ( 1)
wherein ko is the distribution coefficient, Co is the impurity concentration of the original aluminum, and fs is the ratio of the solid fraction. The above equation shows that when the distribution coefficient ko is smaller than 1, the impurity concentration Cs of the solid fraction is smaller than the impurity concentration Co of the original aluminum and that when the distribution coefficient ko is larger than 1, the impurity concentration Cs of the solid fraction is greater than the impurity concentration Co of the original aluminum. Since eutectic impurities are smaller than 1 in distribution coefficient, the solid fraction prepared from aluminum containing such impurities is lower than the original aluminum in impurity concentration. Accordingly when the original aluminum is melted and thereafter solidified, high-purity aluminum can be obtained advantageously by selectively separating off the resulting pro-eutectic aluminum. Segregation solidification processes based on this principle are already known as disclosed, for example, in U.S. Pat. No. 3,671,229, No. 3,211,547, etc. However, peritectic impurities are larger than 1 in distribution coefficient, so that aluminum containing such impurities affords a solid fraction which is higher than the original aluminum in impurity concentration. Accordingly when aluminum containing both eutectic impurities and peritectic impurities is treated by the conventional segregation solidification process to obtain pro-eutectic aluminum selectively, the resulting solid fraction is lower in the concentration of eutectic impurities but higher in the concentration of peritectic impurities than the original aluminum.
Aluminum containing both eutectic impurities and peritectic impurities is therefore usually treated by adding boron to a melt of such aluminum in a container to cause the boron to react with Ti, V, Zr and like peritectice impurities and form insoluble metallic borides such as TiB.sub.2, VB.sub.2, ZrB.sub.2, etc., allowing the mixture to stand for a long period of time, for example, for more than 1 hour to settle the metallic borides, placing the molten aluminum into another container as separated from the borides, and selectively drawing off pro-eutectic aluminum only from the container. This process is inefficient since the settlement of the metallic borides requires much time.